The Maramuni Arc of Papua New Guinea: A Record of Continental Collision, Orogenesis and Arc Magmatism

Abstract:

Although Papua New Guinea is relatively young in geological terms, it lies in a complex tectonic setting of oblique convergence between the Pacific and Australian plates, and trapped between the converging Ontong Java Plateau of the Pacific plate and the Australian continent. This complex setting combined with a lack of regional quantitative datasets has led to uncertainty in the timing and nature of major terrane forming events. The Maramuni arc represents the only continuous record of the tectonic evolution of Papua New Guinea during the Miocene, and hence provides an opportunity to gain insight into subduction dynamics, orogenesis and crustal processes that operated throughout this dynamic period. We present an integrated U-Pb geochronology, Hf isotope and geochemical investigation of the Maramuni arc utilizing a suite of intrusive rocks from the eastern Papuan Highlands that span the Late Miocene from ca. 12 Ma to 6 Ma. Northward dipping subduction beneath Papua New Guinea and early Maramuni arc activity is disrupted by arrival of the Australian continent at the trench from 12 Ma. The magmatic rocks formed from ca. 12–9 Ma have compositional affinities of subduction-zone magmas, but record increasing incompatible trace element contents and decreasing εHf with time, which we interpret to reflect a progressive increase in the crustal component of the magmas. Subsequent porphyry suites emplaced at 7.5-6 Ma are distinct from the older magmatic rocks by their marked HREE-depletion, which reflects a dramatic shift in arc-mantle dynamics. This is interpreted as break-up of the subducted slab and crustal delamination, and is marked by a second phase of uplift in the New Guinea Orogen. These results provide insights into the processes of continent collision and the expression of such processes in the geological record with potential applications in ancient convergent margins.